Question

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How to pass an event to a method?

Is it possible to pass an event as a parameter to a method?

For example, the following method subscribes to the event, does work, and unsubscribes from the event:

void SubscribeDoAndUnsubscribe<TElement, TEventArgs>(
        IEnumerable<TElement> elements,
        ??? elementEvent)
    where TEventArgs: EventArgs
{
    EventHandler<TEventArgs> handler = (sender, e) => { /* Handle an event */ };

    foreach (var element in elements)
    {
         // Subscribe somehow
         element.elementEvent += handler
    }

    // Do things

    foreach (var element in elements)
    {
         // Unsubscribe somehow
         element.elementEvent -= handler
    }
}

Client code:

var elements = new [] { new Button(), new Button() };
SubscribeDoAndUnsubscribe(elements, ??? /* e => e.Click */);

If it’s not possible, how do I achieve the similar logic in other ways? Shall I pass pair of delegates for subscribe/unsubscribe methods?

Answer 1

You have in fact discovered that events are not “first class” in C#; you cannot pass around an event as data. You can pass around a delegate to a method associated with a receiver as a first-class object by making a delegate. You can pass around a reference to any variable as a (mostly) first-class object. (I say “mostly” because references to variables cannot be stored in fields, stored in arrays, and so on; they are highly restricted compared to other kinds of data.) You can pass around a type by obtaining its Type object and passing that around.

But there is no way to directly pass around as data an event, property, indexer, constructor or destructor associated with a particular instance. The best you can do is to make a delegate (or pair of delegates) out of a lambda, as you suggest. Or, obtain the reflection object associated with the event and pass that around, along with the instance.